Pulp and process for pulping

ABSTRACT

The invention relates to a new pulp, which is derived from lignocellulosic material subjected to agitation in an aqueous tetra-alkylammonium salt solution under microwave irradiation. The invention relates also to a process for pulping lignocellulosic material and to a process for softening lignocellulosic material. The treated material is preferably wood, softwood or hardwood.

FIELD OF THE INVENTION

The present invention is directed to a new pulp, which is derived fromlignocellulosic material subjected to agitation in aqueoustetra-alkylammonium salt solution under microwave irradiation. Theinvention is also directed to a process for pulping lignocellulosicmaterial and to a process for softening lignocellulosic material.

BACKGROUND ART Pulp

Pulp is the raw material for the production of paper, paperboard,fiberboard, and similar manufactured products. In purified form, it is asource of cellulose for rayon, cellulose esters, and other cellulosederived products.

Pulp is obtained from plant fiber and is, therefore, a renewable source.Fibrous plants have been used a source for writing materials, e.g.,papyrus, since the earliest Babylonian and Egyptian civilizations. Theorigin of papermaking, which is the formation of cohesive sheet from therebonding of separated fibers, has been attributed to Ts'ai-Lun in Chinain 105 AD, who used bamboo, mulberry bark, and rags. The use of wood asa source of papermaking was not commercially applied until themid-1800s. The principal wood-pulping processes in use today, e.g., thegroundwood, soda, SO₂, or acid sulfite, and the sulfate or kraftprocesses were developed in 1844, 1853, 1866 and 1870, respectively.Since their development, the basic processes have been modified andadapted and the technology has been highly refined.

As with most industries, the environmental and energy concerns of the1970s effected large changes in the operation of pulp and paper mills aswell as much research effort to develop the most energy-efficient andcleanest methods for the production. In most cases, the practical resultfor the short term has been add-on methods, e.g., scrubbers,precipitators, holding ponds, etc., which minimize the discharge ofeffluents. Other trends have been the increasing use of high yield pulpsby modifying the ground wood processes to improve pulp quality, the useof more of the tree in harvesting and chipping, and elimination orminimization of malodorous sulfur compounds in pulping and the toxic andcorrosive chlorine compounds from bleaching.

Before pulping processes, the wood material is treated by harvesting,barking, chipping and screening processes. The purpose of chipping forpulping is to reduce the wood to a size that allows penetration anddiffusion of the processing chemicals without excessive cutting ordamage to fibers. The chips, which are ca 20 mm long, are fairlyfree-flowing and can be transported pneumatically or on belts and thenstored in piles or bins. Kirk-Othmer, Encyclopedia of ChemicalTechnology, 3^(rd) edition, pages 379-391.

The present pulps can be subdivided into mechanical and chemical pulps.

Said mechanical pulps are subdivided into groundwood pulp,thermomechanical pulp (TMP) and chemithermomechanical pulp (CTMP).Groundwood pulp is prepared by pressing wet wood against a wettedrotating grindstone, with the axis of the wood parallel to the axis ofthe wheel. Temperatures in the immediate grinding zone can be 180-190°C. The movement of the water and the removal of pulp controls anddissipates the heat, thus preventing charring of the wood material.After such treatment, the groundwood pulp contains a considerableproportion of (70-80 wt-%) of fiber bundles, broken fibers, and fines inaddition to the individual fibers. The fibers are essentially wood withthe original cell-wall lignin intact. They are, therefore, very stiffand bulky and do not collapse like the chemical-pulp fibers. Sincegroundwood pulps are obtained in yields of ca 95%, their cost isrelatively low. The main direct cost other than wood is power, which isca 49-75 kJ (11,7-17,9 kcal)/ton for normal paper grades.

Thermomechanical pulp (TMP) is prepared by presteaming the wood chips to110-150° C. in order to make them malleable. A thermoplasticization ofthe wood occurs when it is heated above the glass transition point ofwet lignin. When these chips are fiberized in a refiner at highconsistency, whole individual fibers are released; separation occurs atthe middle lamella and a ribbon-like material is produced from the S₁layer of the cell wall. The amount of fibrillization depends on therefining conditions and is critical to the properties of the pulp. Thismaterial has a high light-scattering coefficient, although it is lowerthan that of groundwood, and is highly flexible, which gives goodbonding and surface smoothness to the paper. The increased proportion oflong fibers improves the tearing properties of TMP-pulps, but the fibersin this fraction are stiff and contribute little to bonding. There ismuch less fiber fragmentation than in groundwood pulps.

Chemithermomechanical pulp (CTMP) is prepared in the same manner as TMPbut the chips are pretreated by a mild treatment with sodium sulfite atpH 9-10. In the process, the chips are impregnated with the chemicals,steamed to 130-170° C. and subsequently refined. The yield is 90-92%,which is 2-3% lower than in TMP. A range of properties can be obtainedby adjusting processing variables but in general, CTMP pulp has greaterlong-fiber fraction and lower-fines fraction than a comparablethermomechanical pulp. The intact fibers are more flexible than TMPfibers and, consequently, better sheet-forming and bonding propertiesare obtained. CTMP pulping is reported to be particularly suitable forpulping high-density hardwoods.

In chemical pulping, sufficient lignin is dissolved from the middlelamella to allow the fibers to separate with little, if any, mechanicalaction. However, a portion of the cell-wall lignin is retained in thefiber, and an attempt to remove this during digestion would result inexcessive degradation of the pulp. For this reason, ca. 3-4 wt-% oflignin is normally left in hardwood chemical pulps and 4-10 wt-% is leftin softwood chemical pulps. The lignin is subsequently removed bybleaching in separate processing if completely delignified pulps are tobe manufactured.

The concentration of the cooking liquor in contact with the woodinfluences the rate of delignification. Because the time required fordiffusion of the chemical through the wood structure and the depletionof the reagent concentration as it penetrates the chip, delignificationproceeds more slowly at the center of the chip. In order to preventovercooking of the principal portion of the pulp, digestion is normallyhalted before the centers of the larger chips are adequatelydelignified. The resultant pulp thus contains a portion of nondefiberedwood fragments, which are separated by screening and returned to thedigester or fiberized mechanically.

The dominant chemical wood-pulping process is the kraft or sulfateprocess. The alkaline pulping liquor or digesting solution containsabout 3 to 1 ratio of sodium hydroxide and sodium sulfide. The namekraft, which means strength in German, characterizes the stronger pulpproduced when sodium sulfide is included in the pulping liquor, comparedwith the pulp obtained if sodium hydroxide alone is employed, as in theoriginal soda process. The alternative term, i.e., the sulfate processis derived from the use of sodium sulfate as a makeup chemical in therecovery process. Sodium sulfate is reduced to sodium sulfide in therecovery furnace by organic-derived carbon.

Solutions of sodium sulfide and sodium hydroxide are in equilibrium:

H₂O+Na₂S⇄NaHS+NaOH

Aqueous sodium sulfide is therefore a source of hydroxide ions and mustbe considered in adjusting the chemical charge. A system has beendeveloped in the North American industry to put sodium hydroxide andsodium sulfide on an equivalent basis by expressing them both as theirequivalent weight to sodium oxide, Na₂O. The percent of sodium sulfidein the mixture, when both Na₂S and NaOH are expressed as Na₂O, is knownas the sulfidity. The chemical charge, liquor composition, time ofheat-up and time and temperature of reaction are functions of the woodspecies or species mix being digested and the intended use of the pulp.A typical set of conditions for southern pine chips in the production ofbleachable-grade pulp for fine papers is active alkali 18%; sulfidity25%; liquor to wood-ratio 4:1; 90 minutes at 170° C. in the top heatingzone and 90 min at 170° C. in the second zone. Hardwoods require lessvigorous conditions primarily because of the lower initial lignincontent.

Although the kraft process is a highly developed, adaptable, andefficient process, there are some problems and disadvantages for itsuse. Efforts are being made in individual mills to minimize energy,water, and chemical requirements. Additionally, there are two problemsinherent in the chemistry of the process, namely low carbohydrate yieldand the formation of malodorous organic sulfur compounds.

One modification to the kraft process that is being applied commerciallyis the polysulfide process. When elemental sulfur is added to a solutionof sodium sulfide and sodium hydroxide, the sulfur dissolves and forms amixture of complexes with the general formula Na₂S_(x) (where x is 2-5,depending on the equilibrium conditions and how much sulfur is added).Sulfur Na₂S_(x) is an oxidizing agent, which, under the conditions ofkraft pulping, converts the hemiacetal function to a relativelyalkali-stable aldonic acid. The increase in yield in polysulfide processis proportional to the amount of added sulfur to ca 10% based on wood.

One additional pulping method is sulfite pulping. In the originalsulfite pulping process, wood was pulped with an aqueous solution of SO₂and lime. Calcium sulfite has very limited solubility above pH 2, andexcess of SO₂ gas was maintained in the digester in order to keep the pHbelow said level. Thus, the process can be contrasted with the kraft orsoda processes as being an acid process. Currently, bases other thancalcium are used with SO₂ solutions, and sulfite pulping refers to avariety of processes in which the full pH range is utilized for all orpart of the pulping. Magnesium, sodium, and ammonia are used asalternatives to calcium. Magnesium sulfide has decreasing solubilityabove pH 5, but sodium and ammonium sulfites are soluble at pH 1-14.

In addition to previously discussed pulping methods there are somesemichemical pulping methods. The distinctions between semichemical andhigh yield chemical processes are very small and are more a matter ofgradation between the mechanical and full chemical processes. Asemichemical process is essentially a chemical delignification in whichthe chemical processes are stopped at a point where mechanical treatmentis necessary to separate fibers from partially cooked chips. Any knownchemical process can be used to produce semichemical pulp. The pulps,although less flexible, resemble chemical pulps more than mechanicalpulps because they are not dependent on rupture of the fiber wall forbonding. The yield is 60-85% with a lignin content of 15-20%. The ligninis concentrated on the fiber surface.

Microwaves

It is known from the recent literature concerning organic synthesis thatthe reaction times of the organic reactions are remarkable reduced whenthe energy necessary for the occurrence of the reaction is introduced tothe system by using microwave irradiation. The commonly used frequencyfor microwave energy is 2.45 GHz. There is a wide and continuouslyincreasing literature available in the area of using microwavetechniques in organic synthesis. An example of a short summary articleof this topic was published by Mingos in 1994 (D. Michael P. Mingos;“Microwaves in chemical synthesis” in Chemistry and Industry 1. August1994, pp. 596-599). Loupy et. al. have recently published a reviewconcerning heterogeneous catalysis under microwave irradiation (Loupy,A., Petit, A., Hamelin, J., Texier-Boullet, F., Jachault, P., Mathe, D.;“New solvent-free organic synthesis using focused microwave” inSynthesis 1998, pp. 1213-1234). Another representative article has beenpublished by Strauss (C. R. Strauss; “A combinatorial approach to thedevelopment of Environmentally Benign Organic Chemical Preparations”,Aust. J. Chem. 1999, 52, p. 83-96).

Microwaves in Mechanical Pulping

Patent CA 2008526 discloses manufacturing of pulps using microwaveheating of impregnated lignocellulosic material. The impregnation isconducted with state of the art pulping liquor (Na₂SO₃-solution) in thepresence of catalysts and chelating agent. The impregnation of saidchemicals is followed by irradiation of resulting material in amicrowave-transparent digester. This is followed by a separatemechanical refining step. The main advantage of microwave treatment isthe reduction of cooking time and consumed energy.

Scott et al. (TAPPI Fall Technol. Trade Fair, pp. 667-676) have reporteda process for “microwaving logs for energy savings and improved paperproperties for mechanical pulps”. The treatment was conducted as apretreatment for mechanical pulping without any impregnation ofadditional chemicals. The energy consumption in subsequent mechanicalpulping was decreased up to 15% for the highest employed power level.Apparently, the wooden material was softened by the rapid evaporation ofwater and thus, rapid rupture of the lignocellulosic material.

Microwaves in Dissolution of Wood and Cellulose

FI20031156 discloses a microwave-assisted method to dissolvelignocellulosic material in ionic liquids. The dissolution is completeand can be adapted to any kind of lignocellulosic materials, includingsoft- and hard wood. The dissolution must be conducted in substantialabsence of water. The dissolved material components can be separatedfrom the resulting ionic liquid solution.

Rogers et al. published in 2002 a method for dissolution of purecellulose fibers into ionic liquids in the microwave field (Swatloski,R. P.; Spear S. K.; Holbrey, J. D.; Rogers, R. D. Journal of AmericanChemical Society, 2002, 124, p. 4974-4975). Also here, the dissolutionmust be conducted in substantial absence of water.

Other non-derivatizing organic solvents for cellulose are widelydescribed in “Comprehensive Cellulose Chemistry, Volume 1, Wiley-VCH,page 59-67. Amongst other, aqueous solutions of differenttetra-alkylammonium hydroxides have been proved to be efficient solventsfor cellulose. A complete dissolution is achieved readily. Since wateris always present in excess volumes, said solvents are not practical inderivatization of cellulose.

SUMMARY OF THE INVENTION

Pulping is a significant and one of the most energy consuming industriesin the world. Due to the climate change, continuously growingpopulation, and thus energy consumption, there is a great demand fornew, energy-efficient production technologies in all fields of industry.In pulping, elimination or minimization of malodorous sulfur compoundswould be an additional asset.

It is an object of this invention to provide a new pulp material.

Another object of this invention is to provide a process for pulpinglignocellulosic material.

A further object of this invention is to provide a process for softeningthe lignocellulosic material.

Further objects will become apparent from the following description andclaims.

It is known that cellulose can he completely dissolved in said aqueoustetra-alkylammonium hydroxide solution. It is also known that wood canbe dissolved in ionic liquids in substantial absence of water.

When conducting tests to dissolve cellulose in wood material intoaqueous tetra-alkylammonium hydroxide solution in microwave field, itwas surprisingly found that it was not the cellulose but the lignin inwood material that was dissolved in salt solution.

Unexpectedly, the agitation could be conducted in a manner wherein acomplete to substantial delignification took place and celluloseremained intact as bunches of fine, long fibers. The present inventionaccomplishes a new kind of pulp and process for preparing it.

By tuning the salt concentration and agitation time, the delignificationcould be avoided and simultaneously, the wood material was dramaticallysoftened.

Both in delignification (pulping) and in softening of lignosellulosicmaterials, surprisingly short treatment was required in order to achievesaid results. The lignosellulosic material such as wood could be eitherdelignificated or softened already after one minute's agitation inmicrowave field.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention there is provided a new pulp, which pulp isderived from lignocellulosic material subjected to agitation in aqueoustetra-alkylammonium salt solution under microwave irradiation.

The agitation can take place with or without stirring of lignocellulosicmaterial in said solution.

The lignocellulosic material can be virtually any kind oflignocellulosic material. The primary source of fiber for pulp is wood,such as softwood and hardwood. Other sources include straws, grasses,and canes. Pulp fibers can be principally extracted from any vacularplant found in nature, also nonwood sources such as straws, grasses,e.g., rice, esparto, wheat and sabai; canes and reeds, e.g., primarilybagasses or sugar cane; several varieties of bamboo; bast fibers, e.g.,jute, flax, kenaf, linen, ramie, and cannabis; leaf fibers, e.g., agabaor manila hemp and sisal.

Preferably lignocellulosic material is wood, such as softwood andhardwood.

The lignocellulosic material can be in its original form as found innature, or it can be partially processed. In one preferred embodiment ofthe invention, the lignocellulosic material consists of wood chips,i.e., the lignocellulosic wood material has been subjected to barkingand chipping before agitation of said material in aqueoustetra-alkylammonium salt solution under microwave irradiation.

The lignocellulosic material can pre-treated by impregnating water orsaid aqueous tetra-alkylammonium salt solution into the lignocellulosicmaterial.

The content of tetra-alkylammonium salt in aqueous tetra-alkylammoniumsalt solution can be 1-75 wt-%, preferably 5-60 wt-% and most preferably10-40 wt-%. The cation of the tetra-alkylammonium salt is

wherein R¹, R², R³, and R⁴ are independently a C₁-C₃₀ alkyl, C₃-C₈carbocyclic, or C₃-C₈ heterocyclic group, and the anion of the salt canbe halogen, pseudohalogen, perchlorate, C₁-C₆ carboxylate or hydroxide.

Preferably, the anion is chloride or hydroxide, most preferably theanion is hydroxide.

An especially preferred tetra-alkylammonium salt is the salt wherein R¹,R², R³ and R⁴ are independently C₄ alkyl, and the anion of the salt ishydroxide.

When miscible with water, also other organic ionic compounds can beemployed as a salt component when agitating lignocellulosic materialunder microwave irradiation according to the invention. A variation ofsuch ionic compounds, known as ionic liquids is described in F120031156.

The agitation can be carried out at a temperature between 40° C. and270° C., preferably at a temperature between 70° C. and 210° C., andmost preferably between 120° C. and 190° C.

It is also possible to apply pressure when subjecting lignocellulosicmaterial to agitation in aqueous tetra-alkylammonium salt solution undermicrowave irradiation. When applied, the pressure is preferably below 20Bar, more preferably below 10 Bar, and most preferably between 2 Bar and9 Bar.

The agitation time can vary between 1 minute to 24 hours, depending onthe employed salt and concentration thereof, nature and concentration oflignocellulosic material, on the agitation temperature as well aspossibly applied pressure.

The pulp according to the invention can be employed as material for theproduction of paper, paperboard, fiberboard, and similar manufacturedproducts

According to the invention there is also provided a process for pulpinglignocellulosic material, in which process the lignocellulosic materialis subjected to agitation in aqueous tetra-alkylammonium salt solutionunder microwave irradiation in order to establish partial or completedelignification.

In the pulping process, the agitation can take place with or withoutstirring of lignocellulosic material in said solution.

The lignocellulosic material can be virtually any kind oflignocellulosic material. The primary source of fiber for pulp is wood,such as softwood and hardwood. Other sources include straws, grasses,and canes. Pulp fibers can be principally extracted from any vacularplant found in nature, also nonwood sources such as straws, grasses,e.g., rice, esparto, wheat and sabai; canes and reeds, e.g., primarilybagasses or sugar cane; several varieties of bamboo; bast fibers, e.g.,jute, flax, kenaf, linen, ramie, and cannabis; leaf fibers, e.g., agabaor manila hemp and sisal.

Preferably, the employed lignocellulosic material is wood, such assoftwood and hardwood.

The lignocellulosic material can be in its original form as found innature, or it can be partially processed. In one preferred embodiment ofthe invention, the lignocellulosic material consists of wood chips,i.e., the lignocellulosic wood material has been subjected to barkingand chipping before agitation of said material in aqueoustetra-alkylammonium salt solution under microwave irradiation.

The lignocellulosic material can pre-treated by impregnating water orsaid aqueous tetra-alkylammonium salt solution into lignocellulosicmaterial.

In the pulping process, the content of tetra-alkylammonium salt inaqueous tetra-alkylammonium salt solution can be 1-75 wt-%, preferably5-60 wt-% and most preferably 10-40 wt-%. The cation of thetetra-alkylammonium salt is

wherein R¹, R², R³, and R⁴ are independently a C₁-C₃₀ alkyl, C₃-C₈carbocyclic, or C₃-C₈ heterocyclic group; and the anion of the salt canbe halogen, pseudohalogen, perchlorate, C₁-C₆ carboxylate or hydroxide.

Preferably, the anion is chloride or hydroxide, most preferably theanion is hydroxide.

An especially preferred tetra-alkylammonium salt in the pulping processis said salt wherein R¹, R², R³ and R⁴ are independently C₄ alkyl; andanion of the salt is hydroxide.

When miscible with water, also other organic ionic compounds can beemployed as a salt component when in the pulping process according tothe invention. Applicable compounds are exemplified in FI20031156.

In the pulping process according to the invention, the agitation can becarried out at a temperature between 40° C. and 270° C., preferably at atemperature between 70° C. and 210° C., and most preferably between 120°C. and 190° C.

It is also possible to apply pressure when subjecting lignocellulosicmaterial for agitation in aqueous tetra-alkylammonium salt solutionunder microwave irradiation. When applied, the pressure is preferablybelow 20 Bar, more preferably below 10 Bar, and most preferably between2 Bar and 9 Bar.

The agitation time can vary between 1 minute to 24 hours, depending onthe employed salt and concentration thereof, the nature andconcentration of lignocellulosic material, on the agitation temperatureas well as possibly applied pressure.

In pulping process according to the invention, it is advantageous tochoose said parameters in a manner that delignification oflignocellulosic material is partial or complete.

The pulped lignocellulosic material can be employed as material for theproduction of paper, paperboard, fiberboard, and similar manufacturedproducts

According to the invention there is further provided a process forsoftening lignocellulosic material, in which process the lignocellulosicmaterial is subjected to agitation in aqueous tetra-alkylammonium saltsolution under microwave irradiation.

In the softening process, the agitation can take place with or withoutstirring of lignocellulosic material in said solution.

In said softening process, the lignocellulosic material can be virtuallyany kind of lignocellulosic material. The primary source of fiber forpulp is wood, such as softwood and hardwood. Other sources includestraws, grasses, and canes. Pulp fibers can be principally extractedfrom any vacular plant found in nature, also nonwood sources such asstraws, grasses, e.g., rice, esparto, wheat and sabai; canes and reeds,e.g., primarily bagasses or sugar cane; several varieties of bamboo;bast fibers, e.g., jute, flax, kenaf, linen, ramie, and cannabis; leaffibers, e.g., agaba or manila hemp and sisal.

Preferably, the employed lignocellulosic material is wood, such assoftwood and hardwood.

The lignocellulosic material can be in its original form as found innature, or it can be partially processed. In one preferred embodiment ofthe invention, the lignocellulosic material consists of wood chips,i.e., the lignocellulosic wood material has been subjected into barkingand chipping before agitation of said material in aqueoustetra-alkylammonium salt solution under microwave irradiation.

The lignocellulosic material can pre-treated by impregnating water orsaid aqueous tetra-alkylammonium salt solution into lignocellulosicmaterial.

In the softening process according to the invention, the content oftetra-alkylammonium salt in aqueous tetra-alkylammonium salt solutioncan be 1-75 wt-%, preferably 5-60 wt-% and most preferably 10-40 wt-%.The cation of the tetra-alkylammonium salt is

wherein R¹, R², R³, and R⁴ are independently a C₁-C₃₀ alkyl, C₃-C₈carbocyclic, or C₃-C₈ heterocyclic group and the anion of the salt canbe halogen, pseudohalogen, perchlorate, C₁-C₆ carboxylate, or hydroxide.

Preferably, the anion is chloride or hydroxide, most preferably theanion is hydroxide.

An especially preferred tetra-alkylammonium salt in the softeningprocess is said salt wherein R¹, R², R³, and R⁴ are independently C₄alkyl; and the anion of the salt is hydroxide.

When miscible with water, also other organic ionic compounds can beemployed as a salt component when in the pulping process according tothe invention. Applicable compounds are exemplified in FI20031156.

In the softening process according to the invention, the agitation canbe carried out at a temperature between 40° C. and 270° C., preferablyat a temperature between 70° C. and 210° C., and most preferably between120° C. and 190° C.

It is also possible to apply pressure in the softening process accordingto the invention. When applied, the pressure is preferably below 20 Bar,more preferably below 10 Bar, and most preferably between 2 Bar and 9Bar.

The agitation time can vary between 1 minute to 24 hours, depending onthe employed salt and concentration thereof, nature and concentration oflignocellulosic material, on the agitation temperature as well aspossibly applied pressure.

In softening process according to the invention, it is advantageous tochoose said parameters in a manner that lignocellulosic material is onlysoftened, not pulped. Accordingly, no substantial delignification takesplace during the softening process according to the invention. Thelignocellulosic material structure is ruptured and impregnated withaqueous tetra-alkylammonium salt solution in a manner where the energyand/or chemical consumption in subsequent processing steps is decreased.

The softened lignocellulosic material can be employed as material forthe production of paper, paperboard, fiberboard, and similarmanufactured products.

The present invention accomplishes a new pulp, which can be manufacturedin a rapid and energy efficient manner. The degree of delignification istunable and resulting pulp is of high quality consisting of fine, longfibers. The present also accomplishes a process for softeninglignosellulosic material. Said softened, malleable material can then beprocessed further in a more energy efficient manner. Accordingly, thepresent invention results in lower energy consumption and thus,environmental benefits. Also formation of malodorous organic sulfurcompounds is avoided. The employed tetra-alkylammonium salt is arelatively cheap chemical, which is preferably recycled.

EXAMPLES

The following examples describe the invention without limiting saidinvention into examples. In examples 1-10, treated lignosellulosicmaterial were sticks of Finnish softwood cut from 20 mm long wood chips.The sticks were cut parallel to wood lamellas in order to facilitatelong fibers. The original reason to cut the sticks was the restrictingsize of the microwave reactor, namely 5 ml.

Example 1 Treatment of Softwood in 40% Aqueous TetrabutylammoniumHydroxide in Microwave Field—5 Minutes at 170° C.

Approximately 750 mg of softwood sticks were mixed into 4.5 ml ofaqueous 40% tetrabutylammonium hydroxide solution and agitated for 5minutes at 170° C. in a sealed reactor tube equipped with magneticstirring bar.

The agitation resulted in dark brownish solution comprising long fiberfines. Washing with water gave both bunches of detached pale beigefibers as well as completely separate fibers.

Example 2 Treatment of Softwood in 20% Aqueous TetrabutylammoniumHydroxide in Microwave Field—5 Minutes at 170° C.

Approximately 750 mg of softwood sticks were mixed into 4.5 ml ofaqueous 20% tetrabutylammonium hydroxide solution and agitated for 5minutes at 170° C. in a sealed reactor tube equipped with magneticstirring bar.

The agitation resulted in dark brownish solution comprising long fiberfines. Washing with water gave both bunches of detached pale beigefibers as well as completely separate fibers. The pulp composition wasslightly more intact compared to that of example 1.

Example 3 Treatment of Softwood in 10% Aqueous TetrabutylammoniumHydroxide in Microwave Field—5 Minutes at 170° C.

Approximately 750 mg of softwood sticks were mixed into 4.5 ml ofaqueous 10% tetrabutylammonium hydroxide solution and agitated for 5minutes at 170° C. in a sealed reactor tube equipped with magneticstirring bar.

The agitation resulted in dark brownish solution comprising long fiberfines. Washing with water gave both bunches of detached pale beigefibers and the pulp composition more intact compared to that of example2.

Example 4 Treatment of Softwood in 10% Aqueous TetrabutylammoniumHydroxide in Microwave Field—30 Minutes at 170° C.

Approximately 750 mg of softwood sticks were mixed into 4.5 ml ofaqueous 10% tetrabutylammonium hydroxide solution and agitated for 30minutes at 170° C. in a sealed reactor tube equipped with magneticstirring bar.

The agitation resulted in dark brownish solution comprising long fiberfines. Washing with water gave both bunches of detached pale beigefibers and the pulp composition resembled that of example 1.

Example 5 Treatment of Softwood in 40% Aqueous TetrabutylammoniumHydroxide in Microwave Field—5 Minutes at 120° C.

Approximately 750 mg of softwood sticks were mixed into 4.5 ml ofaqueous 40% tetrabutylammonium hydroxide solution and agitated for 5minutes at 120° C. in a sealed reactor tube equipped with magneticstirring bar.

The agitation resulted in dark brownish solution comprising long fiberfines. Washing with water gave both bunches of detached pale beigefibers as well as completely separate fibers. The pulp compositionresembled to that of example 2.

Example 6 Treatment of Softwood in 5% Aqueous TetrabutylammoniumHydroxide in Microwave Field—5 Minutes at 120° C.

Approximately 750 mg of softwood sticks were mixed into 4.5 ml ofaqueous 5% tetrabutylammonium hydroxide solution and agitated for 5minutes at 120° C. in a sealed reactor tube equipped with magneticstirring bar.

The agitation resulted in brownish solution comprising some woodensticks and long fiber fines. Washing with water gave dramaticallysoftened sticks of wood, some fines detached from said sticks.

Example 7 Treatment of Softwood in 40% Aqueous TetrabutylammoniumHydroxide in Microwave Field—5 Minutes at 80° C.

Approximately 750 mg of softwood sticks were mixed into 4.5 ml ofaqueous 40% tetrabutylammonium hydroxide solution and agitated for 5minutes at 80° C. in a sealed reactor tube equipped with magneticstirring bar.

The agitation resulted in brownish solution comprising some woodensticks and long fiber fines. Washing with water gave dramaticallysoftened and partially detached sticks of wood, some fines being alsodetached from said sticks.

Example 8 Treatment of Softwood in 10% Aqueous TetrabutylammoniumHydroxide in Microwave Field—30 Minutes at 80° C.

Approximately 750 mg of softwood sticks were mixed into 4.5 ml ofaqueous 10% tetrabutylammonium hydroxide solution and agitated for 30minutes at 80° C. in a sealed reactor tube equipped with magneticstirring bar.

As in example 7, the agitation resulted in brownish solution comprisingsome wooden sticks and long fiber fines. Washing with water gavedramatically softened and partially detached sticks of wood, some finesbeing also detached from said sticks.

Example 9 Treatment of Softwood in 10% Aqueous TetrabutylammoniumHydroxide in Microwave Field—1 Hour at 70° C.

Approximately 750 mg of softwood sticks were mixed into 4.5 ml ofaqueous 10% tetrabutylammonium hydroxide solution and agitated for 1hour at 70° C. in a sealed reactor tube equipped with magnetic stirringbar.

Here, the agitation resulted in transparent, pale brownish solutioncomprising some wooden sticks. Washing with water gave dramaticallysoftened and partially detached sticks of wood.

Example 10 Comparative Treatment of Softwood in 40% Sodium Hydroxide(NaOH) in Microwave Field—5 Minutes at 170° C.

Approximately 750 mg of softwood sticks were mixed into 4.5 ml ofaqueous 40% sodium hydroxide (NaOH) solution and agitated for 5 minutesat 170° C. in a sealed reactor tube equipped with magnetic stirring bar.

Here, the agitation resulted in destruction of fiber material givingslimy, brownish pieces of organic material with non-fibrous properties.

Example 11 Comparative Treatment of Softwood in 40% AqueousTetrabutylammonium Hydroxide Without Microwave Field

Approximately 2000 mg of softwood sticks were mixed into 12 ml ofaqueous 40% tetrabutylammonium hydroxide (NaOH) solution and agitatedfor overnight at 95° C. in a flask equipped with magnetic stirring bar.

Also here, the agitation resulted in destruction of fiber materialgiving slimy, brownish pieces of organic material with non-fibrousproperties.

1. A pulp prepared by a process comprising subjecting a lignocellulosicmaterial to agitation in an aqueous tetra-alkylammonium salt solutionunder microwave irradiation.
 2. The pulp according to claim 1, whereinthe lignocellulosic material is softwood or hardwood.
 3. The pulpaccording to claim 1, wherein the content of tetra-alkylammonium salt inthe aqueous tetra-alkylammonium salt solution is 1-75 wt-%.
 4. The pulpaccording to claim 1, wherein the cation of the tetra-alkylammonium saltis

wherein R¹, R², R³, and R⁴ are independently a C₁-C₃₀ alkyl, C₃-C₈carbocyclic, or C₃-C₈ heterocyclic group; and the anion of thetetra-alkylammonium salt is halogen, pseudohalogen, perchlorate, C₁-C₆carboxylate, or hydroxide.
 5. The pulp according to claim 4, wherein R¹,R², R³, and R⁴ are independently C₄ alkyl; and the anion is hydroxide.6. The pulp according to claim 1-4, wherein the agitation is carried outat a temperature between 40° C. and 270° C.
 7. A process for pulpinglignocellulosic material, comprising subjecting a lignocellulosicmaterial to agitation in aqueous tetra-alkylammonium salt solution undermicrowave irradiation.
 8. The process according to claim 7, wherein thelignocellulosic material is softwood or hardwood.
 9. The processaccording to claim 7, wherein the content of tetra-alkylammonium salt inthe aqueous tetra-alkylammonium salt solution is 1-75 wt-%.
 10. Theprocess according to claim 7, wherein the cation of thetetra-alkylammonium salt is

wherein R¹, R², R³, and R⁴ are independently a C₁-C₃₀ alkyl, C₃-C₈carbocyclic, or C₃-C₈ heterocyclic group; and the anion of thetetra-alkylammonium salt is halogen, pseudohalogen, perchlorate, C₁-C₆carboxylate, or hydroxide.
 11. The process according to claim 10,wherein R¹, R², R³, and R⁴ are independently C₄ alkyl; and the anion ishydroxide.
 12. The process according to claim 7, wherein the agitationis carried out at a temperature between 40° C. and 270° C. 13-18.(canceled)
 19. The process according to claim 7, wherein the process iseffective to establish partial or complete delignification.